SpecialBlock: don't pass blank expiry to wfTimestamp
Without this, providing no wpExpiry in the URL means you get the current
datetime :-P
Bug: T368583
Follow-Up: Ibc648117e6d4981c84e9df9f933ba033d339e908
Change-Id: I809af2f5ee82aad2e85bfee7499bfadee870312e